With downsizing of electronic equipment such as cellular phone, much higher integration and more multilayer metallization of large-scale integrated circuits (LSI) have been rapidly promoted recently. On that account, a multi-pin mounting process for mounting LSI on a substrate in order to mount the LSI on electronic equipment is necessary, and there has been paid attention bare chip mounting using a tape automated bonding (TAB) method or a flip chip method. In such a multi-pin mounting process, it is necessary to precisely arrange on a LSI chip a protruded electrode called a bump that is a connecting terminal.
In order to obtain various precision parts such as a bump, precision fine processing technology is necessary, and the main stream of this technology is photo application at present. The photo application is a general term of technology for manufacturing various precision parts, said technology comprising applying a radiation-sensitive resin composition to a surface of a workpiece to form a film, patterning the film by photolithography and carrying out chemical etching, electroetching and electroforming (mainly electroplating) singly or in combination using the resulting pattern as a mask.
More specifically, a bump is produced by the following procedure. First, a barrier metal that becomes an electrically conductive layer is laminated on a wafer having been worked as a LSI device, and then a radiation-sensitive resin composition, i.e., so-called a resist, is applied and dried. Subsequently, the resist is irradiated with radiation (referred to as “exposure” hereinafter) through a mask so as to form an opening corresponding to the place where a bump is to be formed, and then the resist is developed to form a pattern. Thereafter, using the pattern as a mold, electroplating is performed to deposit an electrode material such as gold or copper. Then, the resin portion is stripped, and the barrier metal is removed by etching. Thereafter, a square chip is cut out from the wafer, followed by a step of mounting such as packaging (e.g., TAB), a flip chip method or the like.
With regard to a shape of the bump, there are bumps of various shapes, such as ball bump, mushroom bump, straight bump, etc., and with regard to a height of the bump, the main stream of conventional bumps is a bump having a height of not less than 15 μm, more specifically about 20 to 30 μm.
The present inventors have already proposed a radiation-sensitive resin composition which can be preferably used for photo application (see patent document 1). If this composition is used, formation of a resin film having a film thickness of 20 to 30 μm is possible, and it becomes possible to form a bump having a height of about 20 to 30 μm. Moreover, it is also possible to form a highly precise bump because the composition has been improved in adhesion to a base in the pattern developing step, wettability by a plating solution and resistance to a plating solution.
By the way, in the case where a LSI chip is connected to a substrate through a bump as described above, a stress is caused by a difference in coefficient of linear thermal expansion between the LSI chip and the substrate, and the LSI chip and the substrate are sometimes disconnected from each other. To inhibit such connection failure and to enhance reliability of the device, it is desirable to increase the amount of gold or solder of the bump that is a connecting portion to thereby relax the stress. In this case, if the distance between bumps (also referred to as a “pitch” hereinafter) is relatively large as in the conventional devices, the amount of gold or solder can be secured by increasing the area of the gold or solder portion. However, if the pitch is narrowed with the demand for bumps of high precision, the area of the gold or solder portion cannot be increased, and hence, it becomes necessary to increase the height of the bump to secure the amount of gold or solder.
In recent years, therefore, there is a marked tendency to highly precisely form a bump having a greater height (also referred to a “high bump” hereinafter) on a LSI chip (also referred to as a “chip” simply hereinafter) by photo application. However, if a radiation-sensitive resin composition that is used for usual bump formation is used, it is difficult to form a resin film having a film thickness necessary for forming a high bump on a chip base. Even if a resin film of such a film thickness can be formed, the portion near the base is not sufficiently cured by irradiation with radiation because of low radiation transmittance, though the surface layer is cured, and as a result, a desired pattern cannot be obtained with high precision.
The present inventors have already proposed a radiation-sensitive resin composition that is preferable for forming such a high bump. By the use of this composition, a resin film having a sufficient film thickness, e.g., a dry film thickness of not less than 50 μm, can be formed on a chip substrate, and therefore, this composition can be preferably used for forming a high bump. Moreover, because this resin composition has excellent radiation transmittance, the resin film composed of the composition can be sufficiently cured even at the portion near the substrate. Accordingly, a desired pattern can be obtained with high precision by photolithography (see patent document 2).
In the case where a high bump is formed using a resin film composed of the above composition, however, a plating solution sometimes comes into the interface between the substrate and the resin film in the plating step because the adhesion between the resin film and the substrate in the developing step is not sufficient. Moreover, because of insufficient wettability of the resin film by a plating solution, plating defects sometimes take place.
Patent document 1: Japanese Patent Laid-Open Publication No. 39709/2000
Patent document 2: Japanese Patent Laid-Open Publication No. 241372/2003